1. Field of the Invention
The present invention relates to the bleaching of plant materials destined for nutritional consumption by means of hydrogen peroxide in an alkaline aqueous solution to enhance the whiteness thereof.
Exemplary plant materials which can be bleached according to the invention include brans of oil plants such as sunflowers, brans of cereals such as wheat, maize, oats, barley or whole cereal grains; it is also envisaged to bleach pulps originating from various products such as beet or sugar beet, citrus fruit, for example lemon, fruits such as pears, peaches, apples, apricots, oil plants such as sunflowers, soya beans, cereals, and pulps which originate from the extraction of the fraction which usually enhances the value of such products, namely, sugar, fruit juice, pectin, oil or starch.
2. Description of the Prior Art
Plant materials of the above type are typically treated with alkaline aqueous solutions containing hydrogen peroxide for purposes of "delignifying" and bleaching them.
Such bleached plant materials have a high plant fiber content and can be used, in particular, for producing edible food products and, especially, low-calorie food products, for example dietary flours.
However, the food products obtained from these fibers have too dark a color to permit them to be used as flour substitutes.
A number of techniques have been proposed to this art to obviate this shortcoming. Thus, French Patent No. 2,382,866 (Bayer) describes the bleaching of a beet pulp suspension by treatment with an alkaline hydrogen peroxide solution at a pH of 9.5 to 10, followed by filtration, washing and drying. U.S. Pat. No. 4,649,113 to Gould describes the treatment of plant fibers for nutrition, in suspension at room temperature, with an alkaline hydrogen peroxide solution having a pH maintained between 11.2 and 11.8. This treatment at ambient temperature and at a controlled pH permits effective "delignification." The treatment of plant materials as described in the above patents, solely with an alkaline hydrogen peroxide solution, does not enable products of satisfactory whiteness to be obtained as a result of the rapid decomposition of the peroxide on contact with impurities contained in the plant fibers.
To obtain plant materials having a higher degree of whiteness, French Patent No. 2,647,641 describes subjecting the plant material to an acid pretreatment, followed by washing, prior to carrying out the bleaching treatment.
EP-A-337,653 describes pretreating the plant material with an alkaline solution, followed, if appropriate, by washing, before adding the hydrogen peroxide. The pretreatment is conducted in the absence of hydrogen peroxide and at a pH of between 10 and 13 and a temperature of greater than 25.degree. C. For the bleaching phase, the plant material is maintained in contact with the peroxide solution at a pH of 9.5 to 11.0 at a temperature of greater than 25.degree. C., for a sufficiently long time to produce bleached fibers. The bleached material is neutralized with an acid before drying.
This process permits lignocellulosic plant materials to be converted into useful fibrous cellulosic products, in particular into low-calorie materials which are suitable for human consumption.
The alkaline pretreatment permits a large number of impurities to be eliminated, but entails yellowing of the lignocellulosic material of the plant fiber.